Ultra-low Carbon Bainitic Steel Research Articles

Effect of Microstructure on Corrosion Resistance of Ultralow‐Carbon Bainitic Steel

The effect of microstructure on the corrosion behaviors of ultralow‐carbon bainitic (ULCB) steel in 3.5 wt% NaCl solution is discussed. Four ULCB steel specimens with different microstructures are designed via controlling the cooling routes after austenization, namely, furnace cooling (FC), air cooling (AC), oil cooling (OC), and water cooling (WC). Their corrosion behaviors are investigated by alternating immersion test (including weight loss and rust layer observation) and electrochemical tests (including polarization curves and electrochemical impedance spectroscopic measurements). Results show that the corrosion resistance of ULCB steel specimens decreases with increase of cooling rate, that is, in the following sequence: FC > AC > OC > WC. Furthermore, the electron backscatter diffraction characterization and scanning Kelvin probe force microscopy are performed to build the relationship between microstructure and corrosion behaviors of ULCB steels. The reduced corrosion resistance of ULCB steels results from the increased number of microgalvanic couples because of the significantly increased density of grain boundaries with increase of cooling rate.

The Role of Cold Rolling Reduction on the Microstructure and Mechanical Properties of Ultra-Low Carbon Bainitic Steel.

The present study investigates the microstructure and mechanical properties of ultra-low carbon bainitic steel (UCBS) under different cold rolling reductions. When the rolling reduction ratios were increased to 80%, the microstructure was refined, and the lath width of the bainite decreased from 601 nm to 252 nm. The ultimate tensile strength and yield strength increased from 812 MPa and 683 MPa to 1195 MPa and 1150 MPa, respectively, whereas the elongation decreased from 15.9% to 7.9%. In addition, the dislocation density increased from 8.3 × 1013 m−2 to 4.87 × 1014 m−2 and a stronger γ-fiber texture was obtained at the 80% cold rolling reduction ratio. The local stress distribution and kernel average misorientation were not uniform and became more severe with increased rolling reduction ratios. The strength increment of UCBS was primarily due to boundary strengthening and dislocation strengthening. The theoretical strength increment agreed well with the experimental measurements, which can be helpful for the design and production of UCBS for broad engineering applications.

Effect of Welding Heat Input on Microstructure and Properties of Coarse Grain Zone in Heat Affected Zone of Ultra-Low Carbon Bainitic Steel

In order to explore the effect of different the welding heat input on microstructure and impact properties in coarse grain zone in heat affected zone (CGHAZ) of ultra-low carbon bainitic steel welded joint. In this paper, Gleeble-3500 thermal simulation testing machine was used to simulate different heat input, to study the effect of linear energy on the microstructure and impact toughness of the CGHAZ of Q420qEN steel joint. The scanning electron microscope (SEM), oscillographic impact microscope and transmission electron microscope (TEM) were used. The results show that the microstructure of CGHAZ is mainly the lath bainite and granular bainite. When the welding heat input is 30kJ•cm-1, the CGHAZ grains have obvious growth phenomenon. With the increase of the welding heat input, the grain size also grows. Meanwhile, as the welding heat input increases from 18kJ•cm-1 to 30kJ•cm-1, the impact toughness of the CGHAZ of joint increases first and then decreases at -20°C.

Characterization of Nanoscale Mn/Fe-Rich Intermetallic B2-NiAl in Ultra-Low Carbon Bainitic Steel with Super-High Strength

Characterization of Nanoscale Mn/Fe-Rich Intermetallic B2-NiAl in Ultra-Low Carbon Bainitic Steel with Super-High Strength

Microstructural engineering and strength-impact toughness prediction in ultra-low carbon bainitic steel

ABSTRACTThe strength–toughness combination of an ultra-low carbon, low alloyed ‘air-cooled’ steel has been enhanced by developing and refining the granular bainite (GB) structure. Suitable alloy design and thermo-mechanical treatment involving forging, austenitisation (950–1150°C) and air-cooling resulted in the formation of GB matrix along with small islands of martensite–austenite constituents and allotriomorphic ferrite grains. Microstructural characterisation and phase transformation behaviour were studied by scanning electron microscopy and electron backscattered diffraction analysis. The effect of microstructural parameters on the strength-impact toughness combination was investigated. The steel showed an exceptionally low yield ratio, which is beneficial for seismic resistance application.

Weldability of 1000 MPa Grade Ultra-low Carbon Bainitic Steel

Maximum hardness test in weld heat-affected zone (HAZ), oblique Y-groove cracking test and mechanical property test of welding joint of 1000 MPa grade ultra-low carbon bainitic steel were carried out, so as to research the weldability of the steel. The results show that the steel has lower cold cracking sensitivity, and preheating temperature of 100 °C can help completely eliminate cold cracks, generating good process weldability. The increase of preheating temperature can reduce the hardening degree of heat-affected zone. The strength of welding joint decreases and hardness reduces when heat inputs increase, and excellent mechanical properties can be obtained when low welding heat inputs are used. Fine lath bainites of different orientations combined with a few granular bainites that effectively split the original coarse austenite grains are the foundation of good properties.

High strength steel (600–900 MPa) deposited metals: microstructure and mechanical properties

The microstructure and mechanical properties of the deposited metals of five metal cored wires for welding 600–900 MPa ultralow carbon bainitic steels were investigated. It was found that lowering the C content and optimising the Mn, Ni, Mo and Cr contents of the deposited metal significantly improved the strength and toughness. At the lowest strength level (627 MPa), ferrite dominated, mainly acicular ferrite and proeutectoid ferrite. However, at the highest strength level (>800 MPa), bainite dominated, mainly degenerate upper bainite and granular bainite. Good strength toughness match was achieved by a deposited metal having an interlaced multiphase microstructure with degenerate upper bainite and martensite acting as main strengthening phases, and granular bainite and acicular ferrite acting as toughening phases.

Influence of Inclusions on Early Corrosion Development of Ultra-Low Carbon Bainitic Steel in NaCl Solution

The early corrosion development of ultra-low carbon bainitic (ULCB) low alloy steel in NaCl solution was studied by ex situ imaging of corrosion morphology and in situ monitoring of microarea current density and potential, and the corrosion mechanism from initial localized corrosion to uniform corrosion was interpreted. The results indicate that the corrosion development of ULCB steel from initial localized corrosion around inclusions to the uniform corrosion on the whole steel surface is controlled by the galvanic couple effect between different phases resulting from their electrode potential difference in electrolyte solution. The early localized corrosion of steel matrix is initiated and accelerated by the galvanic couple effect between MnS inclusions and steel matrix to form the initial corrosion gaps and the circular corrosion spots around inclusions. The ohmic drop caused by solution resistance influences the acceleration effect of the galvanic couple. With the separation of inclusion from steel matrix, this galvanic couple effect becomes invalid, which results in the expansion from localized corrosion to uniform corrosion. The microgalvanic couple between martensite/residual austenite (M/A) islands and bainite ferrite also accelerates the anodic dissolution of bainite ferrite phase; however, its acceleration corrosion effect is much weaker than that caused by MnS inclusion.

Charpy Impact Properties of Grain Boundary Allotriomorphic Ferrite and Granular Bainite Duplex Microstructure

A new type of high strength and low cost bainitic steel with ultra-low carbon content and high Si content has been developed on the basis of Mn-series air-cooling bainitic steels. The tensile properties of YS>690MPa and the impact toughness of AKV>60J at-40°C were obtained by controlling the processing parameters. This was attributed to the formation of the grain boundary allotriomorphic ferrite (FGBA) and the granular bainite (GB) with different shape of M/A islands. The high strength due to the inter-lath lamellar M/A islands or retained austenite companying with high dislocated bainitic ferrite laths of average 300nm width. The effect of microstructure on the impact crack initiation and propagation was studied. The results showed that crack initiation occurred in two different types of sites: at interphase boundaries of bainite ferrite (BF) and M/A islands, at grain boundaries. The FGBA and bainite ferrite (BF) both had blunting effect on microcrack tip to reduce the crack propagation path. Because of the presence of FGBA, the unit crack path was short, at less than 5μm. The blunting effect of BF could be enhanced by the M/A islands, which force the cracks change the propagation path and reduce the unit crack path to less than the size of bainite packets. The mechanism of low temperature microcrack origin of the ultra-low carbon bainitic (ULCB) steel with the microstructure of the FGBA and GB was also discussed.

The Evolution of M/A in Ultra-Low Carbon Bainite Steel

The amount, size and structure of M/A in ultra low carbon bainitic steels，which are mainly affect by the transformation process, the cooling rate and the tempering treatments, have been investigated. The different cooling rate induce the variation of distribution of amount and size of M/A. There are bigger M/A mass with harder phase at low cooling rate while smaller M/A with less hard phase at high cooling rate. A series tempering tests have been carried out, which indicating that the tempering treatment has significant effect on the evolution of M/A.

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bainitic steel during continuous cooling. Continuous cooling transformation (CCT) curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln[1/(1 −f)] and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth patterns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agreement with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.

Improved properties of Mn series ultralow carbon steel with ultrafine bainitic structure

In order to reduce the cost of alloying elements, Mn series ultralow carbon bainitic (ULCB) steels have been developed. The different cooling conditions have been used to improve the mechanical properties of Mn series ULCB steels in the present study. The hot rolled ULCB steel plates with 26 mm thickness were water quenched to different finishing quenching temperatures (550, 450 and 350°C) followed by air cooling. Results show that the as rolled ULCB steel plates exhibit high yield strength of 775 MPa and low energy transition temperature of −55°C when the finishing quenching temperature is chosen at 450°C. Low temperature impact toughness was improved through control of the finishing quenching temperature. Microstructure observations show that a fine granular bainite structure is obtained, and the granular bainite structure exhibited shorter crack propagation path than both granular ferrite and lath martensite structures, which contributed to the improved combination of strength and toughness. Furthermore, the formation process of fine granular bainite structure was investigated.

The corrosion resistance of ultra-low carbon bainitic steel

The corrosion resistance of ultra-low carbon bainitic (ULCB) steel was compared with a weathering steel 09CuPCrNi through accelerated corrosion tests. The results indicated that the corrosion resistance was almost the same for ULCB and 09CuPCrNi based on the weight loss. It can be seen that the grain refinement did not deteriorate corrosion resistance property. The homogeneous microstructure, lower carbon content and random distributing ∑3 boundary could effectively increase the corrosion resistance of the ULCB steel. The characteristics of the rust layers indicated that the inner rust layer contained nanocrystalline Fe 3− x O 4 particles, while copper and chromium alloying additions were enriched at the rust layer and substrate interface in ULCB steel. These factors played important roles in forming a compact protective rust layer.

A Study on the Microstructure of Ultra Low Carbon Bainitic Steels by RPC Technique

The effect of tempering process on the microstructure of ultra low carbon bainitic (ULCB) steel produced by relaxation precipitation controlled phase transformation (RPC) has been investigated by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The results showed that the final microstructure mainly contained lath-like bainitic ferrite, granular bainite and martensite-austenite (MA) constituent in ULCB steels. On tempering at 650°C a slight increase was detected in the effective grain size as the strain-induced precipitates pinned up the dislocation walls and subgrains. After tempering at 700°C, bainitic ferrite laths started to coarsen and polygonal ferrite occurred. The effective grain size of ULCB steels in as-rolled condition was 1.5 μm at the tolerance of 10o~15o measured by EBSD technique.

Study on laser welded heat-affected zone in new ultralow carbon bainitic steel

800 MPa grade ultralow carbon bainitic (NULCB) steel is the recently developed new generation steel, which was produced by thermo mechanical controlled processing & relaxation-precipitation controlling transformation (TMCP&RPC) technique. The microstructure and the mechanical properties of the heat-affected zone (HAZ) in NULCB steel under laser welding conditions were investigated by using a Gleeble-1500 thermal simulator. The experimental results indicate that the simplex microstructure in the HAZ is granular bainite that consists of bainite-ferrite (BF) lath and M-A constituent when the cooling time from 800 to 500°C (t 8/5) is 0.3–30 s, and the M-A constituent consists of twinned martensite and residual austenite. As t 8/5 increases, the hardness and tensile strength of HAZ decreases, but they are higher than that of the base metal, indicating the absence of softened zone after laser welding. The impact toughness of HAZ increases at first and then decreases when t 8/5 increases. The impact energy of HAZ is much higher than that of the base metal when t 8/5 is between 3 and 15 s. It indicates that excellent low temperature toughness can be obtained under appropriate laser welding conditions.

Processing Low and Ultra-Low Carbon Bainitic Steels with Excellent Property Combinations

Six experimental low and ultra-low carbon C-Mn-Mo-Nb-B and one conventional TMCP steel heats have been prepared to study the effects of chemical composition and hot deformation on the microstructure and the strength-toughness properties. In physical simulation tests, it was found that the deformation of austenite below the non-recrystallization temperature enhances the formation of higher-temperature bainitic morphologies and polygonal ferrite. On the other hand, hardness exhibits relatively low sensitivity to the degree of deformation below Tnr, whereas the deformation results in a distinct refinement in the microstructures, as determined by SEM-EBSD measurements, suggesting an improvement in the impact toughness. Simultaneous alloying with Mo-Nb-B seemed to be most efficient to provide high hardness and strength. Hot rolling trials indicated that the yield strength in the range 500-700 MPa with the excellent toughness down to –80 °C can be achieved in low carbon (≈ 0.03%) bainitic grades.

Effect of austenite deformation and chemical composition on the microstructure and hardness of low-carbon and ultralow-carbon bainitic steels

The effect of hot deformation and chemical composition on the microstructure and hardness of experimental grades of low-carbon and ultralow-carbon C-Mn-Mo-Nb-B steels is studied by the method of physical simulation. It is shown that deformation of austenite at a temperature below that of recrystallization leads to formation of polygonal ferrite and high-temperature ferrite with bainitic morphology. The effect is the most pronounced in boron-bearing steels. Simultaneous alloying with Mo, Nb, and B seems to be the most effective for ensuring high hardness.

Comparison of Cold Formability of Cold Drawn Non-heat-treated Steels Having Similar Strength

The cold formability of the drawn non-heat-treated steels, i.e. dual phase (DP) steel, low Si steel and ultra low carbon bainitic (ULCB) steel, was examined in terms of the deformation resistance and the forming limit. The present investigation was aimed at elucidating the effect of drawing on the cold formability of non-heat-treated steels which is directly affected by drawing since no heat treatment are involved during forming processes for them. A special care was taken for the present steels to exhibit the similar strength after drawing, so eliminating the strength effect. The present steels after drawing revealed the elastic-near perfect plastic behavior in compression. After drawing, the low Si steel exhibited the lowest deformation resistance estimated by the absorbed energy during deformation. In case of the forming limit in terms of the critical strain under which no cracking occurs during the upsetting test of the drawn steels, the low Si steel and the ULCB steel were better than the conventional heat-treated steel. Accordingly, among several non-heat-treated steels which can replace the conventional heat-treated steel as forging steels, the low Si steel seems to exhibit the best performance if they have the similar strength. The compressive deformation behavior of the present drawn non-heat-treated steels was discussed in association with the strain hardened state and the Bauschinger effect developed by drawing. In addition, their cold formability was explained by the plastic incompatibility between the constituent phases of each steel.

Balancing metallurgy and production of high strength steels

The ever increasing demands for strip with high strength and strain values and tolerances stretch the capabilities of production sites to their limits, because the high strength at room temperature has to be combined with limited loads imposed on the mill during hot rolling. The ultralow carbon bainitic steels developed to meet customers' demands have chemical compositions that extensively retard static recrystallisation. Therefore a double hit compression technique was used to quantify the static softening behaviour. High speed plane strain compression (PSC) experiments were performed to simulate actual multistand rolling schedules at semi-industrial speeds in combination with a controlled coiling simulation. The sample size allows determination of mechanical properties, microstructure, or crystallographic texture. The force–displacement curves permit more accurate tuning of the rolling models. Hence, control of thickness at the head end of the strip during hot rolling can be improved. The transformation behaviour was measured using dilatometry and thermal analysis. In combination with the mechanical properties obtained from the PSC samples, these data allowed conclusions to be drawn regarding optimum cooling rate and coiling temperature. This subsequently allowed high productivity industrial production of high strength steels having good mechanical properties, while reducing loads in the mill to acceptable levels. The work has demonstrated that the combination of metallurgical models and experiment is a powerful tool in rapid implementation of product developments.

The effect of prior compressive deformation of austenite on toughness property in an ultra-low carbon bainitic steel

For the purpose of investigating the trend of toughness versus variation of microstructural constituents for an experimental ultra-low carbon bainitic steel, the experiments (with and without prior compressive deformation) have been carried out on a Gleeble 1500 machine. The Charpy impact specimens were prepared from the samples treated by the Gleeble machine. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical metallographs and transmission electron micrographs have been examined. The result shows that the prior compressive deformation of austenite promotes the formation of intragranular non-parallel plates of acicular ferrite but stifles the formation of sheaf-like parallel plates of bainitic ferrite. Furthermore, the study emphasizes that the microstructure containing a high volume fraction of acicular ferrite possesses better toughness and strength than the microstructure containing mainly bainite.